Our goal is to identify the chemical processes responsible for the HCN and HNC gas phase abundances in L1157-B1, before and during the passage of the shock. The different studies of H\(_2\) O, NH\(_3\) , CH\(_3\) OH, H\(_2\) CO, HC\(_3\) N, H\(_2\) S, NO, PN, PO by our group [1]}, [2]}, [3]}, [4]}, [5]}, [4]}, [7]}, [8]}, [9]} have led to a coherent picture of L1157-B1, in which the detected molecular emission is accounted for by a C-type shock propagating into a pre-shock medium of density \(n(\hbox{H${}_2$})\simeq 5\times 10^4\hbox{\hspace{1.99997pt}cm$^{-3}$}\)  and shock velocity \(V_s\)  in the range \(20-40\hbox{\hspace{1.99997pt}km\hspace{1.99997pt}s$^{-1}$}\) . We have adopted the same methodological approach as in our previous studies, and have used the public time dependent gas-grain chemical and parametrised shock model UCLCHEM code [3]} to follow the evolution of the gas phase composition across a protostellar shock, adopting a cosmic ray ionization rate of
\(\zeta =1.3\times 10^{-16}\hbox{\hspace{1.99997pt}s$^{-1}$}\) , in agreement with [11]}. We refer the reader to the release paper of UCLCHEM for a detailed description of this public code [3]}. Briefly, the code is run in two phases, where Phase I forms a dense core out of a diffuse, essentially atomic medium. An initial density of \(100\hbox{\hspace{1.99997pt}cm$^{-3}$}\)  is adopted here, for the diffuse medium. During this phase, gas-phase chemistry, freezing on to dust particles and subsequent surface processing, occurs.
